1. Field of the Invention
The present invention relates to motor control systems in general and in particular to motor control systems wherein a single motor controller is time shared among multiple stepper motors.
2. Background of the Invention
Stepper motors are commonly used in manufacturing, industrial processes and as components in a variety of electro-mechanical devices. The motors are used to perform a range of functions from automating processes to positioning work items relative to processing stations. Usually, a plurality of motors are required to effectuate a desired result. For example, in a point-of-sale printer and other paper handling devices, several stepper motors are used to drive customer receipts, journals, document paper feed mechanisms, etc.
A stepper motor is an electro-mechanical device which includes a rotor and a stator. The stator carries a plurality of windings which can be energized singly or in groups to rotate the rotor a precise increment of a complete cycle. The precise increment is used to produce useful work.
If a single motor is used, a motor controller is required to generate current which energizes the motor windings. If multiple motors are used, which is usually the case, the motor contorller must not only generate the current that energizes the windings, but it must also select the proper motor whose windings or coils are to be energized. The prior art provides several types of motor controllers which can be used to control multiple stepper motors.
U.S. Pat. No. 4,742,287 provides a motor controller for a printer. The motor controller consists of a CPU which outputs simplex selection signals on several of its output pins. The simplex signal on each of the related output pins selects one of a plurality of motors. The selected motor is then energized by drivers which are connected to other output pins of the microprocessor. The drawback with this controller is that it requires dedicated simplex lines and associated circuitry for selecting a motor. In order to minimize cost, CPUs which are used as motor controllers have a limited number of output pins which provide the motor selection signals and the motor drive signals. If too many of the pins are used to provide motor selection signals, there will not be enough pins to provide the drive signals and visa versa. The net result is that the simplex technique of selecting a motor is not economical and/or practical.
U.S. Pat. No. 4,055,786 describes a motor control system wherein a motor controller generates a plurality of drive signal which are through individual diode arrays to energize the coils of a selected motor. Motor selection signals are fed from separate output terminals of a command unit computer. The drawback with this system is that each motor requires a separate diode array for activating its coils and a simplex line for its selection.
U.S. Pat. No. 3,792,332 describes a control interface unit consisting of a plurality of address lines, control lines and decoders to address a particular device and device driver. A control box or computer generates control signals which energize the respective lines. This technique requires several lines from the computer and a fair amount of external logic which interface the computer to the motors. As discussed above, the majority of the computer controllers do not have enough output lines and output pins to use the described technique.
U.S. Pat. No. 3,486,093 describes a motor control system consisting of a computer, stepper motor selection circuits and a plurality of drive circuits. Each motor receives energization and selection signal from dedicated circuits.
U.S. Pat. No. 3,448,361 describes a control system wherein separate loads are connected through separate SCR and bandpass filters and separate decoupler circuits to a common power supply line. The bandpass filters capture control signals which are generated and outputted on the power lines.
U.S. Pat. No. 3,416,053 describes a control circuit with different sets of drivers and different motor select lines for controlling a plurality of motors.